Field of the Invention
The present invention is related to a control technique and a stimuli generating technique in connection with visual evoked responses. More particularly, the present invention is related to stimuli generating methods, devices and control systems for inducing visual evoked potentials from human viewers using imperceptible flickering multi-color lights.
Description of Related Art
In recent years, with the progress of medical engineering, brain computer interface (BCI) based on steady-state visual evoked potential (SSVEP) responses are widely applied. By the “steady-state visual evoked potential (SSVEP)” technique, the visual nerves of a human viewer's brain are stimulated with signals continuously flickering at a fixed frequency so as to induce the brain to generate electroencephalographic (EEG) signals corresponding to the flickering signals at the frequency. The generated EEG signals are referred to as steady-state visual evoked potential (SSVEP) signals, which may also be referred to as visual evoked responses. For the conventional technologies, the SSVEP signals are more reliable than the other types of visual evoked potential (VEP) and have a higher information transfer rate (ITR). Thus, the SSVEP technique has become a technique with quite considerable potential for development and can be applied to disability aids fields (including the driving of wheel chairs or for the dialing of mobile phones).
However, in the technique, the flickering signals used to induce the EEG signals generally employ flickering light sources with a mono wavelength and at a lower frequency (e.g., lower than 15 Hz) as stimuli sources for the human viewer's brain. The reason is that the frequency of the flickering light sources is inversely proportional to signal intensity of the SSVEP signals. That is to say, when the flickering light sources are at a lower frequency, signal intensity of the SSVEP signals will be stronger as to be detected easily. Nevertheless, when the frequency of the flickering light sources become further lower, visual fatigue or discomfort may be caused to the viewer due to the flickering lights, and even worse, diseases, such as migraine and seizure attacks of epilepsy, may be caused to the viewer. Therefore, if the technique has to be extended to the application in everyday life for ordinary people, it may be difficulty acceptable for the market due the aforementioned reasons.
Moreover, the flickering signals to induce the EEG signals typically use mono-color and simple patterns, such as checkerboard grid patterns or block graphic patterns, as visual stimuli patterns for the viewer. Therefore, the patterns provided by the conventional techniques can not be combined to display videos and images viewed by the ordinary viewers and thus, have difficult to encode the visual stimuli patterns into display images. Further, the visual stimuli patterns have lower flickering frequencies, which leads to distortion of encoded images and have to be calibrated with additional digital image processing techniques.